Stax SRM-700S
Stax Pro-standard electrostatic driver unit with balanced circuit design, built to power the brand's high-end electrostatic headphones.
Description
The SRM-700S is a dedicated amplifier for Pro-standard electrostatic headphones, operating on a principle distinct from conventional headphone amplifiers: it does not simply amplify an audio signal, it also supplies the bias voltage required for Stax electrostatic transducers to function. It forms the central element of an electrostatic chain, between the source and the headphones.
Its output stage is built around a fully discrete dual FET, paired with a low-noise current emitter follower circuit. This technical choice limits residual hiss and aims for detailed signal reproduction. The output stage operates without negative feedback, using very low-noise field-effect transistors, and relies on a fully balanced circuit construction throughout. A large heatsink dissipates the heat generated by the power transistor array.
The SRM-700S delivers a maximum output voltage of 450 Vrms at 1 kHz, with a Pro bias voltage of 580V, and offers a gain of 60 dB for a harmonic distortion of 0.01% or less. The frequency response extends from DC to 100 kHz when paired with an SR-009S. Volume and balance are adjusted via a four-gang potentiometer with electrical shielding, intended to limit interference along the signal path.
In terms of connectivity, the unit offers a switchable input, balanced via three-pin XLR connectors or unbalanced via RCA connectors, as well as an unbalanced RCA output. It also has two headphone outputs, allowing two electrostatic headphones to be connected simultaneously or two models to be compared without changing the connection.
This driver unit is aimed at owners of Pro-standard Stax electrostatic headphones seeking a dedicated power source, capable of making the most of transducers such as the SR-009S. Its balanced design and carefully engineered output stage make it a sound choice for demanding listening in a fixed setup.
Technical specifications
| Frequency response | DC - 100 kHz (with an SR-009S) |
| Gain | 60 dB |
| Harmonic distortion | 0.01% or less (1 kHz / 100 Vrms output) |
| Input impedance | 50kΩ (RCA), 50kΩ x 2 (XLR) |
| Maximum output voltage | 450 Vrms (1 kHz) |
| Bias voltage | PRO 580V |
| Dimensions (W x H x D) | 240 x 103 x 393 mm |
| Weight | 6.3 kg |
What to check before buying used
The Pro bias voltage (580V) means the exact compatibility of the connected electrostatic headphones must be checked, as an incorrect pairing could damage the transducers. Inspect the RCA and XLR connectors as well as the proprietary Stax headphone socket, which are prone to oxidation and loose contacts after several years. The large heatsink and power transistors in the output stage should show no signs of overheating, as prolonged stress can affect the discrete FETs. Finally, listen for the absence of hiss or crackling at the volume potentiometer, which would indicate worn contacts.
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